The present invention is in the field of optical components, more particularly, polymeric optical components. By optical components are meant here, thermo-optical components, electro-optical components or passive components.
Both thermo-optical and electro-optical components are known. The working of thermo-optical components is based on the phenomenon of the optical waveguide material employed exhibiting a temperature dependent refractive index.
Polymeric thermo-optical components generally comprise a polymeric three-layer structure on a substrate. The three-layer structure comprises a low refractive index lower cladding layer, a high refractive index core layer, and a low refractive index upper cladding layer. On top of the upper cladding layer heating elements are provided (usually metal strips) to heat the polymeric cladding and core material, in order to change the refractive index for switching. The working of electro-optical devices is based on the phenomenon of the non-linear optically active material employed exhibiting an electric field dependent refractive index. Polymeric electro-optical components in general also comprise a polymeric three-layer structure. The three-layer structure comprises a low refractive index lower cladding layer, a non-linear optically active, high refractive index core layer, and a low refractive index upper cladding layer. On top of the upper cladding layer electrodes are provided to apply an electric field to the non-linear optically active material to change the refractive index for switching.
Optical components having an at least penta-layered polymer structure on a substrate comprising:
a) a low refractive index lower cladding layer,
b) a core-matching refractive index lower cladding layer,
c) a core layer,
d) a core-matching refractive index upper cladding layer, and
e) a low refractive index upper cladding layer, are also known in the art.
With this specific layer structure optimum transversal confinement can be obtained, which results in less loss of light and an improved switching efficiency.
For optical components preferably silicon substrates are used. These substrates are readily available on the market and are of homogeneous thickness. Furthermore, they are frequently used in integrated circuit techniques and apparatus. One disadvantage of silicon is its high refractive index. Due to this high refractive index the light of the propagating mode might leak into the silicon substrate. The low refractive index lower cladding layer a) is applied to prevent leaking of light from the propagating mode into the silicon substrate. When other substrates are used, the low refractive index lower cladding a) is also of advantage in controlling the confinement of the propagating mode. Using a low refractive index lower cladding a) of appropriate index and thickness gives ample freedom in designing the core-matching refractive index cladding layers b) and d) and the core layer c).
As described above, the optical components usually comprise metal electrodes on top of the upper cladding layer, either for use as heating elements or for applying an electric field. These electrodes are usually made of gold and/or other metals such as chromium, nickel, copper, platinum, or combinations or alloys thereof.
The low refractive index upper cladding e) is applied to prevent leaking of the light from the propagating mode into the attenuating (gold) electrodes. The refractive, indices of the low refractive index lower and upper cladding layers a) and e) are usually (approximately) the same.
Employing a low refractive index upper cladding layer e) with a larger thickness than that of the low refractive index lower cladding layer a) makes it possible to use a core-matching refractive index upper cladding layer d) which is thinner than the core-matching refractive index lower cladding b). In this case the resulting combined thickness of the low refractive index upper cladding d) and the core-matching refractive index upper cladding e) is smaller than the combined thickness of the low refractive index lower cladding a) and the core-matching refractive index lower cladding b). As a consequence, the structure is transversally asymmetric, with the core layer being close to the electrodes and thus experiencing stronger induced thermo-optical or electro-optical effects, resulting in a more efficient component.
The core-matching refractive index lower cladding b) and the core-matching refractive index upper cladding d) are applied to obtain transversal confinement of the propagating mode. The refractive index can be chosen in a relatively wide range to achieve the required properties, such as: monomode behavior, good overlap with a Standard Single Mode Fiber (SMF).
Lateral confinement can be achieved by all known methods for defining channels in planar waveguiding components. Suitable methods are:
1. shaping the core layer by etching techniques (for instance reactive ion etching with oxygen plasma) to obtain a buried channel waveguide,
2. bleaching the core layer to obtain a buried channel waveguide,
3. shaping either of the core-matching refractive index upper and lower cladding layers b) and d) to obtain a ridge (strip loaded) or an inverted ridge waveguide,
4. bleaching either of the core-matching refractive index upper and lower cladding layers b) and d) to obtain a ridge (strip loaded) or an inverted ridge waveguide.
All these techniques are known to the artisan and need no further elucidation here. When using technique 1, the core layer is etched away, leaving only the channel waveguide. Subsequently, core-matching refractive index upper cladding material is applied both on top of the core layer c) and onto the areas where the core material was etched away. This technique and also technique 2 are preferred because they can result in symmetrical channel waveguides. Symmetrical channel waveguides show low polarization dependence of the modal properties. When the bleaching technique is used, the refractive index of the core-matching refractive index cladding layers b) and d) should be adapted to the refractive index of the bleached parts of the core. When the shaping of the core technique is used, the refractive index of the core-matching refractive index upper cladding layer material is chosen such as to give the required properties, such as: monomode behavior, good overlap with a Standard Single Mode Fiber (SMF), low polarization dependence, low bend losses.
The polymers used for thermo-optical devices according to the invention are so-called optical polymers. Many optical polymers are known in the art, but there is still need for improvement. A particular problem of polymeric waveguides is the difference between the refractive indices of the core layer and the surrounding cladding layers. Typically, these index differences are in the range of 0.003 to 0.008. In waveguide switches switching is induced by index differences of 0.001 (digital switches) to 0.0001 (interferometer switches). These small index differences can be induced by thermo- or electro-optical properties of polymeric materials. In a thermo-optical switch the core index is lowered by locally heating the layer stack by means of a heating element. The closer the heater is to the core, the more efficiently this index lowering can be performed by a lower switching power. To prevent unwanted light absorption by the heater elements, it is advantageous to apply a low-index cladding layer between the waveguide and the heating element. The lower the index of the cladding layer is, the thinner this cladding layer can be, while leakage to the heater is still prevented. To prevent excess loss in the cladding layer and leakage to the substrate, this material must have a low absorption at the operating wavelengths (1.3 and 1.5 xcexcm). Low refractive index polymers have been disclosed in WO96/28493, but their optical loss is relatively high. It is therefore an object of the invention to provide polymeric material with very low refractive index, and very low optical loss, preferably less than 0.15 dB/cm. However, the polymeric material must also display high Tg because of chemical and optical stability, and be cross-linkable to obtain cladding layers suitable for thermo-optical waveguides. Moreover, when polymeric material is used as a waveguide core, it is advantageous to use material with an index similar to that of the optical fiber attached to said waveguide, which effective index for standard single mode fiber is 1.467 at 1.3 xcexcm and 1.468 at 1.5 xcexcm. When doped silica is used as a core, the polymers of the invention can be used advantageously as cladding because their refractive indices can be lower than that of the glass core, which has the advantage that these hybrid waveguides can be rendered athermal.
It has now been found that a cross-linkable fluorinated polymer comprising the carbonate moiety having the formula: 
wherein n=1-10 and m=0-9 meets these demands. Preferably n=1-3 and m=0-3.
The various layers can be applied by spin-coating. In order to be able to spin-coat layer-on-layer, it is often necessary to cross-link one layer before applying the next. Therefore, the optical polymers or NLO polymers are rendered cross-linkable either by the incorporation of cross-linkable monomers or by mixing cross-linkers such as polyisocyanates, polyepoxides, etc. into the polymer.
Preferably, the polymer further comprises a cross-linkable moiety derived from a diol selected from: 
wherein
A, A1, and A2 are independently a bond or C1-12 alkylene, or together with the carbon atoms to which they are bonded form a 5- or 6-membered ring;
B is independently O or C1-4 alkyl;
Q is xe2x80x94COxe2x80x94C(=E)D, wherein
D is H or C1-4 alkyl; and
E is C1-6 alkylidene;
and each of the alkyl, alkylene, and alkylidene groups may be halogenated.
More preferably, the cross-linkable moiety is derived from the diol with the formula HOxe2x80x94CH2xe2x80x94CH(OH)xe2x80x94CH2xe2x80x94Oxe2x80x94COxe2x80x94C(xe2x95x90CH2)CH3.
The term C1-4 alkyl means an alkyl group with 1 to 4 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, tert-butyl, and the like. Methyl is the preferred alkyl group.
The term C1-12 alkylene means an alkylene group with 1 to 12 carbon atoms, such as methylene, ethylene, propylene, 2,2,-dimethylpropylene, dodecylene, and the like. A, A1, and A2 are preferably a bond or an alkylene group with 1-3 carbon atoms.
The term C1-6 alkylidene means an alkylidene group with 1 to 6 carbon atoms, such as methylene, ethylidene, propylidene, 2-methylpropylidene, and the like. Methylene and ethylidene are the preferred alkylidene groups.
When the alkyl, alkylene, or alkylidene groups are halogenated, chlorine and fluorine are the preferred halogens. Fluorine is the most preferred halogen. The index of the polymer can be fine-tuned by selecting the number, the type, and the combination of halogens.
The polymer of the invention can be prepared by standard methods known in the art for the preparation of similar polymers. For instance, the bischloroformate of hexafluorobisphenol A or hexafluoroisopropylidene-dicyclohexanediol bischloroformate can be polymerized in suitable solvents with the hexafluoro-perhydro-bisphenol A, the synthesis of which has been disclosed in EP 0,279,462, optionally in the presence of suitable cross-linkable moieties, such as the above-mentioned diols.
Non-linear electric polarization may give rise to several optically non-linear phenomena, such as frequency doubling, Pockels effect, and Kerr effect. In order to render polymeric non-linear optical material active (obtain the desired NLO effect macroscopically), the groups present in the polymer, usually hyperpolarizable side-groups, first have to be aligned (poled). Such alignment is commonly effected by exposing the polymeric material to electric (DC) voltage, the so-called poling field, with such heating as will render the polymeric chains sufficiently mobile for orientation.
In order to enhance the stability of the thermo-optical component, oxygen scavengers and radical scavengers and the like may be added to the optical polymers.
The invention is further illustrated by the following examples.